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mosfet flicker noise


Analysis of 1/f noise in MOSFET circuits is typically performed in the frequency domain using the standard stationary 1/f noise model. Recent experimental results, however, have shown that the estimates using this model can be quite inaccurate especially for switched circuits. In the case of a periodically switched transistor, measured 1/f noise power spectral density (psd) was shown to be significantly lower than the estimate using the standard 1/f noise model. For a ring oscillator, measured 1/f-induced phase noise psd was shown to be significantly lower than the estimate using the standard 1/f noise model. For a source follower reset circuit, measured 1/f noise power was also shown to be lower than the estimate using the standard 1/f model. In analyzing noise in the follower reset circuit using frequency domain analysis, a low cutoff frequency that is inversely proportional to the circuit ontime is assumed. The choice of this low cutoff frequency is quite arbitrary and can cause significant inaccuracy in estimating noise power. Moreover, during reset the circuit is not in steady state and thus frequency domain analysis does not apply. The paper proposes a nonstationary extension of the standard 1/f noise model, which allows us to analyze 1/f noise in switched MOSFET circuits more accurately. Using our model we analyze noise for the three aforementioned switched circuit examples and obtain results that are consistent with the reported measurements.

1/f noise in MOSFETs was of concern mainly in the design of low frequency linear analog circuits such as bias circuits, audio amplifiers, etc.. As CMOS technology scaled down to the submicron regime, 1/f noise has become of greater concern in a wider range of circuit designs. Scaling has enabled the use of CMOS technology in many new applications such as RF circuits and CMOS image sensors. These circuits have been found to be quite sensitive to 1/f noise. , MOSFET 1/f noise power increases rapidly with technology scaling. It is, therefore, becoming more important to accurately estimate the effect of 1/f noise for a wide variety of MOSFET circuits. Analysis of 1/f noise in MOSFET circuits is typically performed using the well established stationary 1/f noise model , which henceforth will be referred to as the standard 1/f noise model. Recent experimental results, however, show that the estimates using this standard model can be quite inaccurate especially for switched circuits. An important class of such circuits is periodically switched circuits, which are widely used in RF applications, such as switched capacitor networks, modulators and demodulators, and frequency converters.

In the simplest case of a periodically switched transistor, it was shown that the measured drain voltage 1/f noise power spectral density (psd) is much lower than the estimate using the standard 1/f noise model. Another example that has recently been receiving much attention is 1/f-induced phase noise in CMOS oscillators . Unlike the amplitude fluctuations, which can be practically eliminated by applying limiters to the output signal, phase noise cannot be reduced in the same manner. As a result, phase noise limits the available channels in wireless communication. Recent measurements show that the 1/finduced phase noise psd in ring oscillators is much lower than the estimate using the standard 1/f noise model.

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